Probing the Lattice Anharmonicity of Superconducting YBa$_2$Cu$_3$O$_{7-\delta}$ Via Phonon Harmonics

TL;DR

This study investigates the non-linear lattice dynamics of YBa₂Cu₃O₇−δ superconductor under strong excitation, revealing anharmonic phonon behavior and its potential impact on non-equilibrium superconductivity.

Contribution

It demonstrates the observation of phonon third harmonics and mode anharmonicity in a high-temperature superconductor, linking lattice vibrations to electronic properties.

Findings

Detection of phonon third harmonic indicating strong anharmonicity

High-amplitude phonons modify electronic charge transfer resonance

Implications for phonon-driven non-equilibrium superconductivity

Abstract

We examine coherent phonons in a strongly driven sample of optimally-doped high temperature superconductor YBa$_2$Cu$_3$O$_{7-\delta}$. We observe a non-linear lattice response of the 4.5\,THz copper-oxygen vibrational mode at high excitation densities, evidenced by the observation of the phonon third harmonic and indicating the mode is strongly anharmonic. In addition, we observe how high-amplitude phonon vibrations modify the position of the electronic charge transfer resonance. Both of these results have important implications for possible phonon-driven non-equilibrium superconductivity.